Light sources such as light emitting diodes (LEDs) can be used, e.g., for backlighting liquid crystal displays (LCDs), street lighting, and home appliances. LEDs offer several advantages over alternative light sources. Among these are greater efficiency and increased operating life.
FIG. 1 shows a schematic diagram of a conventional circuit 100 for driving a light source, e.g., an LED string. FIG. 2 shows a waveform 200 of a current flowing through the LED string in FIG. 1. As shown in FIG. 1, the circuit 100 for driving an LED string 108 includes a power source 102, a rectifier 104, a capacitor 106, a controller 110, and a buck converter 111. The power source 102 provides an input alternating-current (AC) voltage. The rectifier 104 and the capacitor 106 converts the input AC voltage to an input direct-current (DC) voltage VIN.
Controlled by the controller 110, the buck converter 111 further converts the input DC voltage VIN to an output DC voltage VOUT across the LED string 108. Based on the output DC voltage VOUT, the circuit 100 produces an LED current ILED flowing through the LED string 108. The buck converter 111 includes a diode 106, an inductor 118, and a switch 112. The switch 112 includes an N-channel transistor as shown in FIG. 1. The controller 110 is coupled to the gate of the switch 112 via a DRV pin and coupled to the source of the switch 112 via a CS pin. A resistor 114 is coupled between the CS pin and ground to produce a sense voltage indicative of the LED current ILED. The switch 112 controlled by the controller 110 is turned on and off alternately.
Referring to FIG. 2, when the switch 112 is in an ON state, the LED current ILED ramps up and flows through the inductor 118, the switch 112 and the resistor 114 to ground. The controller 110 receives the sense voltage indicative of the LED current ILED via the CS pin and turns off the switch 112 when the LED current ILED reaches a peak LED current IPEAK. When the switch 112 is in an OFF state, the LED current ILED ramps down from the peak LED current IPEAK and flows through the inductor 118 and the diode 106.
The controller 110 can operate in a constant period mode or a constant off time mode. In the constant period mode, the controller 110 turns the switch 112 on and off alternately and maintains a cycle period Ts of the control signal from pin DRV substantially constant. An average value IAVG of the LED current ILED can be given by:
                                          I            AVG                    =                                    I              PEAK                        -                                          1                2                            ·                                                                    (                                                                  V                        IN                                            -                                              V                        OUT                                                              )                                    ×                                                            V                      OUT                                                              V                      IN                                                        ×                                      T                    S                                                  L                                                    ,                            (        1        )            where L is the inductance of the inductor 118. In the constant off time mode, the controller 110 turns the switch 112 on and off alternately and maintains an off time TOFF of the switch 112 substantially constant. The average value IAVG of the LED current ILED can be given by:
                              I          AVG                =                              I            PEAK                    -                                    1              2                        ·                                                                                V                    OUT                                    ×                                      T                    OFF                                                  L                            .                                                          (        2        )            According to equations (1) and (2), the average LED current IAVG is functionally dependent on the input DC voltage VIN, the output DC voltage VOUT and the inductance of the inductor 118. In other words, the average LED current IAVG varies as the input DC voltage VIN, the output DC voltage VOUT and the inductance of the inductor 118 change. Therefore, the LED current ILED may not be accurately controlled, thereby affecting the stability of LED brightness.